1. Field of the Invention
The present invention relates to a method of fabricating a micromechanical structure, and in particular to a method of preventing peeling or cracking between sacrificial layers in the fabrication of a microelectromechanical structure (MEMS).
2. Description of the Related Art
Microelectromechanical structures (MEMS) have found applications in inertial measurement, pressure sensing, thermal measurement, micro-fluidics, optics, and radio frequency communications, and the range of applications for these structures continues to grow. Conventional microelectromechanical structures, such as accelerometers, pressure sensors, flow sensors, microactuators and the like, typically comprise suspended microstructures having a space or gap between a released portion and a substrate and one or more posts attached the substrate.
FIGS. 1-2 are schematic diagrams showing a method for fabricating a suspended microstructure in the related art. FIG. 1 shows a schematic top view of a microelectromechanical structure 100 and FIG. 2 shows a schematic cross section taken along line 2-2 of FIG. 1.
Referring now to FIGS. 1 and 2, the microelectromechanical structure 100 is provided with two different patterned sacrificial layers 104 and 106 sequentially formed on a substrate 102, serving as a mold for forming a microelectromechanical element. Preferably, the patterned sacrificial layers 104 and 106 comprise photosensitive materials such as photoresists, photosensitive polymers or other materials and can be therefore patterned by a method such as photolithography. Fabrication of the patterned sacrificial layers 104 and 106 are described as follows. A first layer of photosensitive material can be blanketly formed on the substrate 102 by methods such as spin coating or similar and is then patterned by photolithography step and a following development, thereby forming the sacrificial layer 104 and exposed portions of the substrate 102. Next, a second layer of photosensitive material can be blanketly formed over the substrate 102, covering the underlying sacrifical layer 104 and the exposed substrate 102. Next, another photolithography step and a following development are performed on the second layer of photosensitive material, thereby forming the sacrificial layer 106 and exposing portions of the sacrificial layer 104 and the substrate 102.
As shown in FIGS. 1 and 2, portions of the sacrificial layer 106 now protrude over a portion of the substrate 102 adjacent to the sacrificial layer 104 and covers portions of the sidewall of the underlying sacrificial layer 104. This may cause cracking or peeling, such as the cracking 110 shown in FIGS. 1 and 2, where the sacrificial layer 106 covering sidewalls of the underlying sacrificial layer 104 meet since stress are induced due to photolithography and development during formation of the sacrificial layer 106. Part of the sacrificial layer 106 adjacent to the cracking 110 may even fall toward a direction 120 illustrated in FIG. 2, thus deforming the shape of the mold 100 and causing deformation of a lateral suspended microstructure (not shown) formed over the microelectromechanical after removal of the sacrificial layers 104 and 106.
Therefore, an improved method of fabricating a microelectromechanical structure is desired.